Some digital-to-analog converter architectures are based on the switching of a certain number of current sources in an output load. For example, the number of current sources switched simultaneously in a converter having an architecture of the thermometric or segmented type is equal to the digital input value to be converted.
FIG. 1 depicts a switch of a known type used in a digital-to-analog converter for making a current source I flow into an output load R. This switch consists of a differential structure comprising bipolar transistors TA and TB controlled by a control signal Uc derived from the digital value to be converted. The function of this switch is therefore equivalent to a switch SW controlled by the control signal Uc.
When the switching transistors TA–TB are switched, the Early effect which is manifested at the transistors must be taken into account for best modeling the dynamic characteristics of the switch. At low frequency, the Early effect is manifested in the form of a resistance Rp known as the Early resistance, with a high constant value between the collector and emitter of the transistors. At a higher frequency, stray capacitive effects of the switch function capacitances in the transistors TA–TB, interconnection capacitances between the elements constituting the switch equivalent to a capacitance Cp put in parallel to the current source I) manifest through an impedance with a value decreasing with the frequency. The Early effect combined with the stray capacitive effects are therefore equivalent to a stray impedance Z having the characteristics of a low-pass filter as depicted in FIG. 2. This stray impedance impairs the linearity of the digital to analog conversion.
Solutions are known for limiting the non-linearity of digital-to-analog converters using switched current sources and exhibiting an Early resistance. In particular, it is possible to insert, between each current source and the switching transistors, an additional stage of the cascode type so as to increase the impedance of the switch seen from its output.
These means for improving the linearity of digital-to-analog converters do however have limitations.
Indeed, this known solution involves associating as many additional circuits as there are switches. A power supply able to output a large current becomes necessary for supplying these additional circuits, which increases the size of the converter and makes it difficult to integrate it in an integrated circuit.
In addition, the stray effects of these many additional circuits interfere with the functioning of the converter overall.
Finally, adding these many additional circuits increases the cost of these digital-to-analog converters.